Fix location of temporary lyx pipes

Make sure that there is a directory separator when creating the path
of the temporary lyx pipes. This changes what is created in the
temporary directory: instead of
├── [drwx------] kile-XXXXXX
├── [drwx------] kile-XXXXXX.lyx
│   ├── [prw-------] lyxpipe.in
│   └── [prw-------] lyxpipe.out
├── [prw-------] kile-XXXXXX.lyxpipe.in
└── [prw-------] kile-XXXXXX.lyxpipe.out
now is it
└── [drwx------] kile-XXXXXX
   ├── [drwx------] .lyx
   │   ├── [prw-------] lyxpipe.in
   │   └── [prw-------] lyxpipe.out
   ├── [prw-------] .lyxpipe.in
   └── [prw-------] .lyxpipe.out
which also leaves no empty kile-XXXXXX.lyx directories at the exit.

This bug was introduced when porting from KTempDir to QTemporaryDir:
KTempDir::name() returns the full path with a trailing slash, while
QTemporaryDir::path() does not have the trailing slash.